In a water wall tube of a sub-critical power generation boiler, boiling phenomenon occurs in which water turns into steam. For such a water wall tube, a rifled tube is used. A rifled tube has a plurality of helical ribs on its inner surface. The plurality of ribs increase the surface area of the inner surface, compared to a steel tube without ribs. Therefore, a rifled tube has an increased contact surface between the inner surface and water, thus improving the power generation efficiency of the boiler.
Further, the plurality of ribs agitate water in the tube, and put the water into a turbulent flow state. Therefore, occurrence of film boiling is suppressed. Film boiling is a phenomenon in which a film-like vapor phase is generated on the inner surface of the tube when the water flowing through the tube is heated and transformed into gas vapor at its boiling point. If film boiling occurs, the tube will be overheated to a high temperature beyond the boiling point, and bursting may occur due to overheating. The plurality of ribs suppress occurrence of film boiling, thereby suppressing bursting due to overheating.
For thermal power generation boilers of recent years, improvement of combustion efficiency and improvement (reduction) of CO2 emission are strongly required. To achieve these improvements, temperature and pressure of steam need to be increased. To realize higher temperature and higher pressure of steam, a high-Cr and high strength rifled tube is required.
International Application Publication No. WO2009/081655 (Patent Literature 1) discloses a method for producing a rifled tube. As disclosed in Patent Literature 1, a rifled tube is generally produced by the following method. First, a steel tube is prepared. A plug having a plurality of helical grooves is attached to a nose of a mandrel so as to be rotatable about the axis of the plug. The plug attached to the mandrel is inserted into the steel tube. By using a die, cold drawing is performed on the steel tube into which the plug has been inserted. Through the above described process steps, the rifled tube is produced.